The clinical importance of detecting phlebothrombosis in hospitalized patients will be evaluated by a technique that could be incorporated readily into routine patient care. I131-labelled fibrinogen will be prepared from a sample of the patients' own plasma by a salting out method followed by Chloramine-T radioiodination. This method permits simultaneous production of several 50 uCi samples of sterile pyrogen-free I131-fibrinogen suitable for intravenous injection in less than 4 hours. A bedside, battery powered, lower extremity scanning radiation detector will be used on each patient daily for 3-6 days after radio-fibrinogen administration to identify thrombosis formation. Hospitalized patients scheduled for elective major surgery and others at increased risk of thromboembolic disease will be selected. Randomly one-half of them will be observed with note being made of evidence for phlebothrombosis and pulmonary embolism. The other half will be surveyed for thrombosis formation by the technique described above; the attending physician of the patient will be informed of any positive findings. The outcome of th two groups will be compared and the type and magnitude of the diferences found will form the basis of judging the potential merit of the procedure for routine patient care.